1. Field of the Invention (Technical Field)
The present invention relates to resin cure monitoring, specifically methods and apparatus for acoustically monitoring physical and chemical changes in a resin during the curing thereof
2. Background Art
Thermosetting resin compositions are increasingly used in various manufacturing processes for the production of structural parts in vehicles and buildings, as well as the production of electronic devices. Resins frequently are cured by heating, whereby the temperature and duration of heating are carefully controlled to optimize curing conditions to promote maximum strength in the final product. Properly managing the curing process by raising or lowering the curing temperature, or carefully timing the termination of heating, ideally is accomplished by monitoring the degree of cure in the resin composition and adjusting the temperature in response thereto. For example, it is desirable to detect precisely when the optimum degree of cure has been realized, so that the application of heat can be terminated before over-curing occurs. To this end, efforts have been made to develop methods and apparatus for monitoring the curing of resin compositions. These efforts have been directed primarily to the development of systems for monitoring resin products directly and in situ, rather than systems for monitoring small samples, the results of which can be correlated to the curing of a product.
One mode for monitoring the curing of a thermosetting resin material is through the propagation of acoustic waves through the resin. Basic acoustic monitoring has previously been accomplished by locating an ultrasonic transmitter against one side of a resinous bulk body, and placing an ultrasonic receiver against the opposite side. Ultrasonic waves are then propagated through the bulk body while it cures. The chemical and physical changes undergone by the bulk body while it cures affect the acoustic waves received by the acoustic receiver. The changes in the waves are monitored in an effort to ascertain the rate and degree of cure. This method of cure monitoring has a considerable margin of error due to the diffusion of the ultrasound waves as they propagate through the medium of a resinous bulk body. The diffusion of the acoustical waves limits the sensitivity of the method to the subtle chemical and physical changes manifested in the resin during curing.
U.S. Pat. No. 4,590,803 to Harrold discloses an acoustic system for monitoring changes in a bulk body during a phase change from liquid to solid. Harrold proposes to improve the sensitivity of acoustical monitoring by disposing a waveguide through the bulk body while the body is still in the liquid state, and then transmitting and monitoring ultrasonic waves through the waveguide while the bulk body solidifies. The margin of error in the monitoring of the cure purportedly is reduced because the waveguide curtails undesirable diffusion of the acoustic signal. The '803 patent to Harrold, however, teaches that the resin cure be monitored using a resinous bulk body of substantial size. Thus, the Harrold device and manner for cure monitoring requires that a substantial volume of liquid resin be poured into a mold or container, the waveguide immersed in the liquid resin, and special provision made for the entry and exit of the waveguide through sealable apertures in the container walls.
U.S. Pat. No. 4,921,415 to Thomas, et al., shows a cure monitoring apparatus having high temperature ultrasonic transducers. The Thomas device requires the use of a mold with special transducer assemblies. The transducers are placed within the mold. When the device is heated, either comparatively expensive, specially designed high temperature transducers must be used or a special transducer cooling system must be employed.
Previous efforts, such as those by Harrold and Thomas, et al., have demonstrated the desirability of acoustical methods for monitoring the curing of resin compositions and compounds. Nevertheless, a need remains for a method and apparatus for acoustically monitoring the curing of a resin, which method and apparatus permits the monitoring to be performed with energy efficiency upon comparatively small quantities of resin, thus obviating the need for special monitoring molds with sealable apertures. Also, a need remains for a method and apparatus for monitoring the cure of a comparatively small quantity of resin, but which uses ordinary, comparatively inexpensive transducer elements to transmit and receive acoustical signals.